1. Technical Field
The present invention relates to spectrometers for identifying analyte materials and more particularly, to a Fourier transform based spectrometer.
2. Description of Related Art
Spectrometers are generally utilized in detecting, characterizing, quantifying, or imaging analyte materials. Such analyte materials may be any chemical substance or object detected, characterized, quantified, or imaged by the spectrometer. For example, the analyte material may include airborne, waterborne and/or surface-adhered chemical species, concealed or buried objects, or facilities suspected to contain chemical or biological species of interest.
One type of conventional spectrometer technology known in the art is the Fourier Transform Infrared (FTIR) Spectrometer. FIG. 1 is a schematic diagram of an FTIR spectrometer known in the art. This type of FTIR spectrometer 100 generally includes an incoherent light source 10, a Michelson interferometer arrangement 30, and an infrared detector 60. The incoherent light source 10 is configured to generate a light beam 12 which is directed to the Michelson interferometer arrangement 30.
In the prior art, the Michelson interferometer arrangement 30 is operatively coupled to the incoherent light source 12 and includes a beam splitter 32, a fixed mirror 34, and a moving mirror 36. The beam splitter 32 is configured to receive the light beam 12 generated by the incoherent light source 10 and to split the light beam 12 into a first light beam 14 and a second light beam 16.
Conventionally, the fixed mirror 34 and the moving mirror 36 are optically coupled to the beam splitter 32. The fixed mirror 34 receives the first light beam 14 and reflects the first light beam 14 parallel to its incident path. Similarly, the moving mirror 36 receives the second light beam 16 and reflects the second light beam 16 along its incident path.
Generally, the moving mirror is capable of being moved between a first position A and a second position B with the movement being restricted to a direction parallel to that of the light beam. Such displacement of the moving mirror generates a variable difference in length between the paths of the reflected first light beam and the reflected second light beam.
The path length difference produces an interference pattern 38 (hereinafter referred to as ‘interferogram 38’) at the beam splitter. More specifically, the moving mirror 36 is translated between position A and position B in precise increments, with a data point being recorded after each translation step. The difference in the varying path lengths of the first light beam and the second light beam results in alternating constructive and destructive interference between these beams, which is mathematically describable as a convolution of the first light beam and the second light beam. Performing a Fourier transform of the interferogram 38 obtained in the time domain produces a frequency domain spectrum of the original light beam.
Thereafter, the beam splitter passes the recombined light beam with superimposed interferogram 18 to an analyte material 50 that is to be analyzed. A detector 60 then detects the radiation pattern and a data processing system 70 generates a spectrogram 80 corresponding to characteristics of the analyte material. The spectrogram may then be compared with other spectrograms corresponding to a plurality of known materials to identify the analyte material.
Conventional FTIR spectrometers, such as the spectrometer described above, suffer from a number of limitations. A major limitation is that such conventional FTIR spectrometers are sensitive to vibrations and thermal effects. More specifically, the moving mirror of the conventional FTIR spectrometers must be positioned with exact precision in order to generate a desired interferogram. Any inaccuracy in the positioning of the moving mirror will degrade or destroy the generated spectrogram.
A further limitation of conventional FTIR spectrometers is that they are unsuitable for long-range or diffraction-limited imaging. More specifically, incoherent light sources used in the conventional FTIR spectrometers are difficult to collimate and propagate over long distances, and cannot be focused to a diffraction-limited spot size.
Additionally, conventional FTIR spectrometers exhibit poor time resolution. This may be attributed to physical limitations on how fast the moving mirror can be accurately translated during generation of the spectrogram. Moreover, under even low-light conditions the spectrograms generated by conventional FTIR spectrometers can be degraded by ambient background radiation, and in bright-light conditions (such as daylight) the ambient background radiation can swamp the signal radiation entirely (signal-to-noise ratio less than 1). Accordingly, there has been a need to develop FTIR spectrometers that overcome drawbacks inherent to conventional FTIR spectrometers.
One example of a FTIR spectrometer developed to overcome the above described drawbacks is provided in U.S. Pat. No. 5,748,309 to Van der Weide et al. Van der Weide teaches the use of use two solid state ultra-short pulsed lasers (USPLs) having variable pulse repetition rates in place of the two mirrors of a Michelson interferometer arrangement. The FTIR spectrometer taught by Van der Weide provides a well behaved beam for imaging but it requires two fragile, expensive, and inefficient solid state USPLs. Further, this type of FTIR spectrometer requires complicated locking electronics to precisely synchronize the repetition rates of the two USPLs.